<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
		<ul id="list">
			<li><button>点击</button></li>
			<li><button>点击</button></li>
			<li><button>点击</button></li>
			<li><button>点击</button></li>
		</ul>
	</body>
</html>

<script type="text/javascript">
	var buttons = document.querySelectorAll('button');
	// 父节点(parentNode) => 点击的时候,给当前的button的父节点添加灰色的下边框
	
</script>
